1. Field of the Invention
This invention relates to a process for improving the fluidity and conveyability of moist Calcium Sulfate precipitates.
2. Description of the Prior Art
Calcium sulfate (gypsum) is a waste product generated in many chemical processes. In phosphoric acid processes, for example, many million tons of gypsum are produced each year. However, because economic reprocessing is possible only in a limited way, most of the gypsum produced is hauled to or must be dumped as environmentally harmful waste into rivers or channels or directly into the ocean.
To an increasing degree, gypsum is generated during the desulfurization of flue gas. Here, the SO.sub.2 in the flue gases of power plants is first bound with lime milk to produce CaSO.sub.3, which is then transformed to gypsum through oxidation.
Although it is possible to process chemically produced gypsum with currently known technology, it is very expensive for several reasons:
1. The material is moister than natural gypsum. In addition to the water of crystallization in the calcium sulfate, the free water content is normally between 5 and 30% by weight. This residual moisture remains in the product because simple filtration or other means of separating the precipitate fail to remove the adhering water without leaving a residue.
2. The flow characteristics of the material are unfavorable. Under the circumstances, it is thixotropic and, therefore, impossible to convey. Discharging the material from silos or other conical bunkers, for example, is hardly possible because masses of this kind behave like "heavy," moist clay whose form can be changed only by means of considerable mechanical effort.
3. As the material is contaminated it can often be used only after special cleaning operations (for example, washing).
Up to now, further processing has been difficult mainly because of the conveying problems mentioned above. Hence, preprocessing steps such as pelletizing or briquetting of the moist gypsum have been developed to facilitate transportation. But such procedures are expensive.
Among the methods of processing phosphoric acid gypsums, to be mentioned are those which neutralize the remaining acid (H.sub.2 SO.sub.4, H.sub.3 PO.sub.4 or HNO.sub.3, etc) with appropriate amounts of Ca(OH).sub.2, CaO or CaCO.sub.3 or with appropriate magnesium compounds. Because the volume fractions of such residual acids are very small, relatively small charges are sufficient. Depending on the amount of gypsum involved, the charges are usually 5% (cf. for example, DE-PS 1 157 128, column 3, lines 49-52 and column 4, lines 16-21 or CS 154 355, according to CA 82, 34518h, (1975).
The disadvantages of the poor flow characteristics of moist di- and semi-hydrate gypsums are hardly reduced by such measures.
It is known from U.S. Pat. Nos. 1,967,959, 2,021,412, 2,063,488 and 2,127,952, that semi-hydrate and anhydrate can be obtained by adding quicklime in amounts of 1/2 to 4 mole of CaO per mole of CaSO.sub.4.2H.sub.2 O (cf. U.S. Pat. No. 2,063,488, page 1, left column, line 41 and U.S. Pat. No. 2,127,952, page 2 left column, line 53) to dihydrate-base natural gypsums. To initiate the process, optionally added is water as (steam), liquid or moist salt admixture. The reaction heat released during hydration of the quicklime serves to remove the water of crystallization from the dihydrate and, in addition, ensures that part of the water evaporates from the moist mixture.
Natural gypsums of this kind usually contain only very small amounts of free moisture, whereas the chemical gypsum, to be processed in accordance with the invention, exhibits moistures that frequently equal the content of the water of crystallization and sometimes even exceed it (cf. page 1, line 21/22).
Through low pressure filtration or centrifuging it is possible to reduce the free water content in chemical gypsum to 20% and--given enough time or high enough centrifuging--to reduce that content even further (cf. example 1, page 8). But no improvement in the fluidity and conveyability can be achieved by this, rather, only a certain degree of energy savings.
Hence, as currently known processes of improving the fluidity and conveyability of chemical gypsums are either inadequate or too expensive, and thus unsatisfactory, a need continues to exist for a simple process which would improve the properties of these bulk products in such a way that difficulties are significantly reduced or eliminated during processing.